Abstract

The efflux of both organic and inorganic anions from plant roots plays an important role in plant nutrition. We are using the patch-clamp technique to investigate anion channel activity in the peripheral cells of Arabidopsis roots which are at the soil:rhizosphere interface. Plant were grown on luxurious sulphate supply (1.7 mM) for 7–10 days and the patch clamp technique was applied to root protoplasts derived from the epidermal cells of the elongation zone and young root hairs. To study sulphate- efflux currents, the pipette was loading with a medium containing 25 mM Cs2sulphate, 1 mM MgATP, 5 mM EGTA, 10 mM HEPES, pH 7.2, and the protoplast bath solution contained 0.5 mM LaCl3, 10 mM CaCl2, 5 mM MgCl2, 10 mM MES, pH 6.0. In the whole-cell configuration, all cells examined showed sulphate efflux currents which were time-dependent, inward rectifying and showed an average maximum peak current density of approximately 30"3 pAypF at approximately y90 mV. These currents could be partially blocked by the anion channel blocker niflumic acid. Reducing the pipette sulphate concentration to 1 mM reduced the sulphate efflux currents to 8"2 pAy pF. These results indicate that sulphate efflux does occur from root cells of Arabidopsis and that this transport is most likely be conducted by anion channels. It has been shown that when plants are grown in luxurious supply of nitrate or sulphate where influx greatly exceeds demand, then efflux from roots can be as high as 70– 80% of the influx (1, 2). The presence of sulphate-efflux channels together with the high influx through sulphate transporters suggests extensive cycling of sulphate at root plasma membrane. Sulphate-efflux channels could play a role in the homeostasis of cellular sulphate concentrations and may limit the usefulness of over-expression of sulphate transporters without a sink for sulphate.